1. Field of the Invention
The present invention relates to a method of forming a pattern of a plurality of densely arranged stripes, and more particularly, to a method of forming a chain of tight-pitched contact patterns (contact chain) by using two exposures processes and one single mask.
2. Description of the Prior Art
In semiconductor manufacturing processes, in order to transfer an integrated circuit layout onto a semiconductor wafer, the integrated circuit layout is first designed and formed as a photo-mask pattern. The photomask pattern is then proportionally transferred to a photoresist layer positioned on the semiconductor wafer. However, with the increasing miniaturization of semiconductor devices, the line widths and lengths become finer and the image quality of the transferred pattern decreases.
Please refer to FIG. 1 and FIG. 2. FIG. 1 is a schematic diagram of a prior art photomask pattern. FIG. 2 is a schematic diagram of the simulation result of relative intensity of light in an exposure process with the photomask of FIG. 1. As shown in FIG. 1, a target pattern 100 includes a plurality of stripe patterns 102. Each stripe pattern 102 has a length of about 160 nm, a width of about 38 nm, and a pitch of about 76 nm. In order to form the target pattern such as pattern 100, a dipole illumination light is usually utilized as the light source during the exposure process. As shown in FIG. 2, by using the dipole illumination light source, the critical dimension (CD) and the resolution of the width can reach a desirable value. However, since the dipole illumination light source only provides a better resolution for width direction, the simulated CD of the length (201.4 nm) still can not meet the predetermined value (160 nm).
In order to make the simulated CD of the length reach the predetermined value (160 nm), a photomask pattern with a reduced length is required. FIG. 3 shows a photomask pattern with a reduced length and FIG. 4 shows the simulation result of relative intensity of light in an exposure process using the photomask of FIG. 3. As shown in FIG. 3, when decreasing the length of the stripe pattern 102, for example, from 160 nm to 100 nm, the simulation result in FIG. 4 reveals that the simulated CD of length (182.8 nm) still can not meet the predetermined value (160 nm) and both the resolutions of the length and the width become worse.
One approach to solve the above-mentioned problem is to make the illumination light source from a dipole shape to a cross-quadrapole shape. Please refer to FIG. 5, which shows the simulation result of relative intensity of light in an exposure process using the photo-mask of FIG. 1. When using the cross-quadrapole illumination light source having both x and y resolutions, as shown in FIG. 5, although the simulated CD of the length can reach the predetermined value (160 nm), however, the resolution of the width is reduced, leading to a poor quality of the image.
Accordingly, a novel exposure method is still needed to form tight-pitched patterns with a desired CD and an improved resolution.